1. Field of the Invention
The invention is related to photonic crystals and to light emitting diodes (LEDs) comprised of multiple-wavelength light sources such as phosphors.
2. Description of the Related Art
By analogy to atomic or molecular crystals, a photonic crystal can be described as a material or system presenting a spatial modulation of its index of refraction or dielectric permittivity. The modulation can be periodic, quasi-periodic, or only possess a short-range order. The periodicity of the lattice, when applicable, which can be one-dimensional (1D), two-dimensional (2D), or three-dimensional (3D), usually scales with the visible to infrared wavelengths for most applications. A distributed Bragg reflector (DBR) is the archetype of the 1D photonic crystal. DBRs present band structures analogous to that of electrons in crystals, with forbidden energies or photonic gaps, where there is no possibility for a photon to propagate. Defects can be introduced in the lattice to form photonic cavities or waveguides (these defects introduce states in the photonic bandgap which allow strongly localized modes, or light propagation, at the corresponding frequency). Photonic crystals have given rise to numerous applications in optoelectronic and photonic integrated devices.
A light emitting diode (LED) is a semiconductor device that emits light when electrically biased in the forward direction, which is known as electroluminescence (EL). An LED is usually comprised of two layers of a semiconducting material. One layer is doped with impurities to make it n-doped (i.e., with mobile electrons), while the other layer is doped with another type of impurities to make it p-doped (i.e., with mobile holes). This forms a structure called a p-n junction. When forward biased, electrons are injected into the junction from the n-region and holes are injected from the p-region. The electrons and holes release energy in the form of photons as they recombine. The wavelength of light, and therefore its color, depends on the bandgap energy of the materials forming the p-n junction. Very thin active layers of smaller band-gap materials, as compared to the p and n layers, referred to as quantum wells (QWs), can be introduced between the p and n layers to greatly increase the overall efficiency of the LEDs and vary the wavelength of emitted light.
Semiconductor materials quality has improved, mainly due to the improvements of their synthesis or growth techniques over the past two decades, namely molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), liquid phase epitaxy (LPE), etc. The external quantum efficiency of semiconductor devices has then greatly improved, and new wavelength ranges have been obtained. Nitride compounds (GaN and related alloys AlGaN and InGaN) are now efficient emitters for violet and blue light, giving ˜30 lumens/watt for commercially available LEDs, while phosphide (AlGaInP) and arsenide (GaAs/AlGaAs) compounds are widely used for red and infrared applications, producing ˜30 lumens/watt for commercially available LEDs. LEDs are nearly as efficient as fluorescent tubes, but only in blue and red wavelength ranges.
The green-yellow portion of the visible spectrum is thus still lacking efficiency, while the combination of different colors on a single substrate (for example, as required in RGB pixels for color display applications, for white light emission, or for any other colored light generation requiring color mixing) is very limited with the semiconductors grown by the conventional methods mentioned previously. Alternative materials should be used, and phosphors can offer good solutions.
A phosphor is a material that can produce light after its excitation via the absorption of energy from an external source. The excitation source may comprise a sufficiently high-energy light beam produced by an LED. The generation of light by the phosphor from absorbed light is called photoluminescence (PL), also referred to as fluorescence.
Phosphors can be made of inorganic materials (garnets with rare earths), light emitting molecules or polymers, or semiconductor nanocrystal quantum dots (NQDs). The II-VI and III-V NQD systems offer both high absorption coefficients (and thus short absorption lengths) in the ultraviolet (UV) and blue ranges, and the possibility of changing the effective index of refraction by changing the matrix in which these NQDs are embedded (for example, SiO2, TiO2, or polymer and resins), without modifying the QD internal emission properties (center wavelength and width of emission band).
In the following, the term “phosphor” is used to represent any type of material performing light conversion, independently of its real nature (inorganic materials, light emitting molecules or polymers, colloidal quantum dots, or any other type of light emitting nanoparticles).
There are two basic combination schemes involving phosphors and LEDs:                (1) Blue-emitting LED combined with phosphors. This solution is currently used to produce white LEDs by combining blue emitting LEDs (in the GaN/InGaN system) with yellow photoluminescent phosphors. Color rendering, mixing, the different directionalities of the LED (directional) and phosphors (isotropic) emissions, and the overall efficiency of such devices are the most important issues related to this scheme.        (2) A UV-emitting LED combined with phosphors. In general, this method can provide both better color rendering and isotropy than (1), or better white equilibrium. However, the overall efficiency is still low.        
These methods can produce a wide range of colors, from the blue to the red, and thus provide a solution for the low-cost fabrication of RGB displays, because phosphors can be positioned precisely to form the different pixels.
FIG. 1 illustrates a conventional EL-PL multi-color structure 100 that includes a phosphor layer 102, also known as secondary emitting species (SES) 102, coating the top of an LED 104, which includes a primary emitting species (PES) 106, a buffer layer 108, a substrate 110 and metal contacts 112 connected to a power supply 114. When forward biased, electrons (e−) are injected into the p-n junction of the PES 106 from the n-region and holes (h+) are injected from the p-region, wherein the electrons and holes cause the PES 106 to release energy in the form of photons 118 as they recombine. The SES 102 are optically-excited 116 after partly or fully absorbing directly extracted light 118 produced by the electrically-pumped PES 106. The absorption by the SES 102 occurs due to the presence of allowed electronic transitions in resonance with the PES 106 photon energies. The SES 102 then re-emits photons 116 of lower energies, or longer wavelengths, upon relaxation to their ground states. This PL by the SES 102 is used for display or lighting, sometimes in combination with the light emitted by the PES 106. This emission scheme is also referred to as light conversion.
For this structure to be highly efficient, the following requirements should be met:                it should be comprised of emitting species with high internal quantum efficiencies (defined as the ratio of the total emitted light intensity to the total absorbed light intensity),        it should maximize the extraction of both PES and SES emissions (possibly in different directions), and        it should maximize the absorption of the PES emission by the SES.        
However, one of the major difficulties with such semiconductor-based structures is caused by the loss of large portions of both PES and SES light emissions due to total internal reflexion (TIR), as shown schematically in FIGS. 2A, 2B and 2C.
FIG. 2A illustrates a device 200 comprised of a semiconductor (dielectric) substrate 202 having planar layers with a high index of refraction (n) and including PES 204. Above the device 200 is an outer medium 206 (air or an epoxy layer) with a low n. Extracted emissions 208 from the PES 204 towards the outer medium 206 are shown, as are lost emissions 210 towards the substrate 202. Total internal reflections (TIR) 212 are also shown, which result in a lowest-order waveguided (WG) mode 214, also known simply as a guided mode, through the thin layers of the substrate 202.
When light is emitted within the planar layers of the substrate 202 with high values of n, only a limited cone of beams can be directly extracted upwards 208 into the outer medium 206. This “escape cone” defines the portion of a solid angle associated with directions of possible direct extraction 208. The escape cone's opening half-angle will be referred to as θc in the following (θc=arcsin(nout/nin)). The higher the difference in indices of refraction, the smaller θc, the narrower the escape cone.
The TIR modes 212, which remain trapped in the optically dense (high index) materials of the substrate 202, are mainly lost, due to internal re-absorption and non-radiative relaxation mechanisms, while sometimes escaping through the sides of the device 200. As WG modes 214, these can represent more than 50% of the overall emission by the PES 204, which are embedded in these high index layers. This loss mechanism is detrimental to the maximization of the extraction of the PES 204 emission.
When phosphors 216 (with a lower index) are placed on top of the optically dense layers 202 (with a higher index), a large part of their emissions 208 is also waveguided 214 inside the high index layers, as shown in FIGS. 2B and 2C, wherein 208a is emitted light directly extracted and absorbed by the phosphors 216, 208b is the PL emitted from the phosphors 216, 214a is a lower-order excitation WG mode and 214b is a higher-order excitation WG mode.
Indeed, electric dipoles, located sufficiently close to a high index layer, always exhibit emissions comprised of evanescent waves, which can efficiently couple to TIR modes. This is shown in the device 300 of FIG. 3A, which includes an electric dipole 302 as a source emitting at λ0, close to a dense planar medium 304 (where nin>nout), i.e., at a distance d typically smaller than λ0, and producing evanescent waves 306, which couple to TIR or WG modes 308 inside the high index layer 304.
FIG. 3B shows dipole emission diagrams for an horizontal and a vertical dipole in a medium with n=1.5 close to a planar cavity of n=2.5. Air is present above the structure, while a substrate with n=1.7 was chosen to be included in the bottom of the structure. The diagrams show the different contributions that combine to produce the dipoles emission: directly extracted light in air (k∥/k0<1), TIR (1<k∥/k0<1.7), and WG modes (k∥/k0>1.7) are present, wherein k0 is the wavevector of light in a vacuum and k∥ is the in-plane component of the wavevector of light for the medium 304. FIG. 3C is a schematic illustrating the multilayer chosen for these simulations.
The closer the emitting dipoles 302 are to the denser planar medium 304, the larger the fraction of evanescent waves 306 (>50% of the overall emission for dipoles located in the vicinity of the interface, d<100 nm), and therefore of the TIR modes in the denser planar medium 304. It should be mentioned that near the interface, the 1D Purcell factor is not negligible and can reach 1.6 for these structures. This factor corresponds to the increase of the spontaneous emission rate kr of the SES, with the internal quantum efficiency given by ηint=kr/(kr+knr) for most light-emitting materials, with knr the non-radiative recombination rate. NQDs can offer sufficiently high absorption coefficients such that the PES light can be absorbed within a few hundreds of nanometers, corresponding to the region of high Purcell factor.
With current state-of-art multi-color LEDs, the TIR or WG light is usually lost, and this accounts in a large part for the limited external efficiency of LEDs. The reduced overall efficiencies cause the devices to overheat, because higher applied voltages are necessary to compensate for the losses, and materials degradation is faster.
Furthermore, the phosphors that are commonly used (rare-earth garnets) are limited by the concentration of emitting ions, implying the LEDs to be coated with thick epoxy and phosphors mixtures (typically 1 mm in height or more), often in the form of a half-sphere to out-couple some TIR modes. These large dimensions in turn imply a reduction of brightness, and obviously of compactness, of the device.
There is a need in the art for improving the far-field patterns of the different components of emission (QWs, phosphors), which make the color rendering angle-dependent: for example, white light LEDs produced by combining blue QWs and yellow phosphors appear bluish in the middle and yellow in the outer regions of the far-field pattern (the color-rendering is not isotropic).
For these reasons, there is a need in the art for improving multi-color LEDs characteristics.